1 2What is Linux Memory Policy?
3 4In the Linux kernel, "memory policy" determines from which node the kernel will
5allocate memory in a NUMA system or in an emulated NUMA system. Linux has
6supported platforms with Non-Uniform Memory Access architectures since 2.4.?.
7The current memory policy support was added to Linux 2.6 around May 2004. This
8document attempts to describe the concepts and APIs of the 2.6 memory policy
9support.
10 11Memory policies should not be confused with cpusets
12(Documentation/cgroups/cpusets.txt)
13which is an administrative mechanism for restricting the nodes from which
14memory may be allocated by a set of processes. Memory policies are a
15programming interface that a NUMA-aware application can take advantage of. When
16both cpusets and policies are applied to a task, the restrictions of the cpuset
17takes priority. See "MEMORY POLICIES AND CPUSETS" below for more details.
18 19MEMORY POLICY CONCEPTS
20 21Scope of Memory Policies
22 23The Linux kernel supports _scopes_ of memory policy, described here from
24most general to most specific:
25 26 System Default Policy: this policy is "hard coded" into the kernel. It
27 is the policy that governs all page allocations that aren't controlled
28 by one of the more specific policy scopes discussed below. When the
29 system is "up and running", the system default policy will use "local
30 allocation" described below. However, during boot up, the system
31 default policy will be set to interleave allocations across all nodes
32 with "sufficient" memory, so as not to overload the initial boot node
33 with boot-time allocations.
34 35 Task/Process Policy: this is an optional, per-task policy. When defined
36 for a specific task, this policy controls all page allocations made by or
37 on behalf of the task that aren't controlled by a more specific scope.
38 If a task does not define a task policy, then all page allocations that
39 would have been controlled by the task policy "fall back" to the System
40 Default Policy.
41 42 The task policy applies to the entire address space of a task. Thus,
43 it is inheritable, and indeed is inherited, across both fork()
44 [clone() w/o the CLONE_VM flag] and exec*(). This allows a parent task
45 to establish the task policy for a child task exec()'d from an
46 executable image that has no awareness of memory policy. See the
47 MEMORY POLICY APIS section, below, for an overview of the system call
48 that a task may use to set/change its task/process policy.
49 50 In a multi-threaded task, task policies apply only to the thread
51 [Linux kernel task] that installs the policy and any threads
52 subsequently created by that thread. Any sibling threads existing
53 at the time a new task policy is installed retain their current
54 policy.
55 56 A task policy applies only to pages allocated after the policy is
57 installed. Any pages already faulted in by the task when the task
58 changes its task policy remain where they were allocated based on
59 the policy at the time they were allocated.
60 61 VMA Policy: A "VMA" or "Virtual Memory Area" refers to a range of a task's
62 virtual address space. A task may define a specific policy for a range
63 of its virtual address space. See the MEMORY POLICIES APIS section,
64 below, for an overview of the mbind() system call used to set a VMA
65 policy.
66 67 A VMA policy will govern the allocation of pages that back this region of
68 the address space. Any regions of the task's address space that don't
69 have an explicit VMA policy will fall back to the task policy, which may
70 itself fall back to the System Default Policy.
71 72 VMA policies have a few complicating details:
73 74 VMA policy applies ONLY to anonymous pages. These include pages
75 allocated for anonymous segments, such as the task stack and heap, and
76 any regions of the address space mmap()ed with the MAP_ANONYMOUS flag.
77 If a VMA policy is applied to a file mapping, it will be ignored if
78 the mapping used the MAP_SHARED flag. If the file mapping used the
79 MAP_PRIVATE flag, the VMA policy will only be applied when an
80 anonymous page is allocated on an attempt to write to the mapping--
81 i.e., at Copy-On-Write.
82 83 VMA policies are shared between all tasks that share a virtual address
84 space--a.k.a. threads--independent of when the policy is installed; and
85 they are inherited across fork(). However, because VMA policies refer
86 to a specific region of a task's address space, and because the address
87 space is discarded and recreated on exec*(), VMA policies are NOT
88 inheritable across exec(). Thus, only NUMA-aware applications may
89 use VMA policies.
90 91 A task may install a new VMA policy on a sub-range of a previously
92 mmap()ed region. When this happens, Linux splits the existing virtual
93 memory area into 2 or 3 VMAs, each with it's own policy.
94 95 By default, VMA policy applies only to pages allocated after the policy
96 is installed. Any pages already faulted into the VMA range remain
97 where they were allocated based on the policy at the time they were
98 allocated. However, since 2.6.16, Linux supports page migration via
99 the mbind() system call, so that page contents can be moved to match
100 a newly installed policy.
101 102 Shared Policy: Conceptually, shared policies apply to "memory objects"
103 mapped shared into one or more tasks' distinct address spaces. An
104 application installs a shared policies the same way as VMA policies--using
105 the mbind() system call specifying a range of virtual addresses that map
106 the shared object. However, unlike VMA policies, which can be considered
107 to be an attribute of a range of a task's address space, shared policies
108 apply directly to the shared object. Thus, all tasks that attach to the
109 object share the policy, and all pages allocated for the shared object,
110 by any task, will obey the shared policy.
111 112 As of 2.6.22, only shared memory segments, created by shmget() or
113 mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy. When shared
114 policy support was added to Linux, the associated data structures were
115 added to hugetlbfs shmem segments. At the time, hugetlbfs did not
116 support allocation at fault time--a.k.a lazy allocation--so hugetlbfs
117 shmem segments were never "hooked up" to the shared policy support.
118 Although hugetlbfs segments now support lazy allocation, their support
119 for shared policy has not been completed.
120 121 As mentioned above [re: VMA policies], allocations of page cache
122 pages for regular files mmap()ed with MAP_SHARED ignore any VMA
123 policy installed on the virtual address range backed by the shared
124 file mapping. Rather, shared page cache pages, including pages backing
125 private mappings that have not yet been written by the task, follow
126 task policy, if any, else System Default Policy.
127 128 The shared policy infrastructure supports different policies on subset
129 ranges of the shared object. However, Linux still splits the VMA of
130 the task that installs the policy for each range of distinct policy.
131 Thus, different tasks that attach to a shared memory segment can have
132 different VMA configurations mapping that one shared object. This
133 can be seen by examining the /proc/<pid>/numa_maps of tasks sharing
134 a shared memory region, when one task has installed shared policy on
135 one or more ranges of the region.
136 137Components of Memory Policies
138 139 A Linux memory policy consists of a "mode", optional mode flags, and an
140 optional set of nodes. The mode determines the behavior of the policy,
141 the optional mode flags determine the behavior of the mode, and the
142 optional set of nodes can be viewed as the arguments to the policy
143 behavior.
144 145 Internally, memory policies are implemented by a reference counted
146 structure, struct mempolicy. Details of this structure will be discussed
147 in context, below, as required to explain the behavior.
148 149 Linux memory policy supports the following 4 behavioral modes:
150 151 Default Mode--MPOL_DEFAULT: This mode is only used in the memory
152 policy APIs. Internally, MPOL_DEFAULT is converted to the NULL
153 memory policy in all policy scopes. Any existing non-default policy
154 will simply be removed when MPOL_DEFAULT is specified. As a result,
155 MPOL_DEFAULT means "fall back to the next most specific policy scope."
156 157 For example, a NULL or default task policy will fall back to the
158 system default policy. A NULL or default vma policy will fall
159 back to the task policy.
160 161 When specified in one of the memory policy APIs, the Default mode
162 does not use the optional set of nodes.
163 164 It is an error for the set of nodes specified for this policy to
165 be non-empty.
166 167 MPOL_BIND: This mode specifies that memory must come from the
168 set of nodes specified by the policy. Memory will be allocated from
169 the node in the set with sufficient free memory that is closest to
170 the node where the allocation takes place.
171 172 MPOL_PREFERRED: This mode specifies that the allocation should be
173 attempted from the single node specified in the policy. If that
174 allocation fails, the kernel will search other nodes, in order of
175 increasing distance from the preferred node based on information
176 provided by the platform firmware.
177 containing the cpu where the allocation takes place.
178 179 Internally, the Preferred policy uses a single node--the
180 preferred_node member of struct mempolicy. When the internal
181 mode flag MPOL_F_LOCAL is set, the preferred_node is ignored and
182 the policy is interpreted as local allocation. "Local" allocation
183 policy can be viewed as a Preferred policy that starts at the node
184 containing the cpu where the allocation takes place.
185 186 It is possible for the user to specify that local allocation is
187 always preferred by passing an empty nodemask with this mode.
188 If an empty nodemask is passed, the policy cannot use the
189 MPOL_F_STATIC_NODES or MPOL_F_RELATIVE_NODES flags described
190 below.
191 192 MPOL_INTERLEAVED: This mode specifies that page allocations be
193 interleaved, on a page granularity, across the nodes specified in
194 the policy. This mode also behaves slightly differently, based on
195 the context where it is used:
196 197 For allocation of anonymous pages and shared memory pages,
198 Interleave mode indexes the set of nodes specified by the policy
199 using the page offset of the faulting address into the segment
200 [VMA] containing the address modulo the number of nodes specified
201 by the policy. It then attempts to allocate a page, starting at
202 the selected node, as if the node had been specified by a Preferred
203 policy or had been selected by a local allocation. That is,
204 allocation will follow the per node zonelist.
205 206 For allocation of page cache pages, Interleave mode indexes the set
207 of nodes specified by the policy using a node counter maintained
208 per task. This counter wraps around to the lowest specified node
209 after it reaches the highest specified node. This will tend to
210 spread the pages out over the nodes specified by the policy based
211 on the order in which they are allocated, rather than based on any
212 page offset into an address range or file. During system boot up,
213 the temporary interleaved system default policy works in this
214 mode.
215 216 Linux memory policy supports the following optional mode flags:
217 218 MPOL_F_STATIC_NODES: This flag specifies that the nodemask passed by
219 the user should not be remapped if the task or VMA's set of allowed
220 nodes changes after the memory policy has been defined.
221 222 Without this flag, anytime a mempolicy is rebound because of a
223 change in the set of allowed nodes, the node (Preferred) or
224 nodemask (Bind, Interleave) is remapped to the new set of
225 allowed nodes. This may result in nodes being used that were
226 previously undesired.
227 228 With this flag, if the user-specified nodes overlap with the
229 nodes allowed by the task's cpuset, then the memory policy is
230 applied to their intersection. If the two sets of nodes do not
231 overlap, the Default policy is used.
232 233 For example, consider a task that is attached to a cpuset with
234 mems 1-3 that sets an Interleave policy over the same set. If
235 the cpuset's mems change to 3-5, the Interleave will now occur
236 over nodes 3, 4, and 5. With this flag, however, since only node
237 3 is allowed from the user's nodemask, the "interleave" only
238 occurs over that node. If no nodes from the user's nodemask are
239 now allowed, the Default behavior is used.
240 241 MPOL_F_STATIC_NODES cannot be combined with the
242 MPOL_F_RELATIVE_NODES flag. It also cannot be used for
243 MPOL_PREFERRED policies that were created with an empty nodemask
244 (local allocation).
245 246 MPOL_F_RELATIVE_NODES: This flag specifies that the nodemask passed
247 by the user will be mapped relative to the set of the task or VMA's
248 set of allowed nodes. The kernel stores the user-passed nodemask,
249 and if the allowed nodes changes, then that original nodemask will
250 be remapped relative to the new set of allowed nodes.
251 252 Without this flag (and without MPOL_F_STATIC_NODES), anytime a
253 mempolicy is rebound because of a change in the set of allowed
254 nodes, the node (Preferred) or nodemask (Bind, Interleave) is
255 remapped to the new set of allowed nodes. That remap may not
256 preserve the relative nature of the user's passed nodemask to its
257 set of allowed nodes upon successive rebinds: a nodemask of
258 1,3,5 may be remapped to 7-9 and then to 1-3 if the set of
259 allowed nodes is restored to its original state.
260 261 With this flag, the remap is done so that the node numbers from
262 the user's passed nodemask are relative to the set of allowed
263 nodes. In other words, if nodes 0, 2, and 4 are set in the user's
264 nodemask, the policy will be effected over the first (and in the
265 Bind or Interleave case, the third and fifth) nodes in the set of
266 allowed nodes. The nodemask passed by the user represents nodes
267 relative to task or VMA's set of allowed nodes.
268 269 If the user's nodemask includes nodes that are outside the range
270 of the new set of allowed nodes (for example, node 5 is set in
271 the user's nodemask when the set of allowed nodes is only 0-3),
272 then the remap wraps around to the beginning of the nodemask and,
273 if not already set, sets the node in the mempolicy nodemask.
274 275 For example, consider a task that is attached to a cpuset with
276 mems 2-5 that sets an Interleave policy over the same set with
277 MPOL_F_RELATIVE_NODES. If the cpuset's mems change to 3-7, the
278 interleave now occurs over nodes 3,5-6. If the cpuset's mems
279 then change to 0,2-3,5, then the interleave occurs over nodes
280 0,3,5.
281 282 Thanks to the consistent remapping, applications preparing
283 nodemasks to specify memory policies using this flag should
284 disregard their current, actual cpuset imposed memory placement
285 and prepare the nodemask as if they were always located on
286 memory nodes 0 to N-1, where N is the number of memory nodes the
287 policy is intended to manage. Let the kernel then remap to the
288 set of memory nodes allowed by the task's cpuset, as that may
289 change over time.
290 291 MPOL_F_RELATIVE_NODES cannot be combined with the
292 MPOL_F_STATIC_NODES flag. It also cannot be used for
293 MPOL_PREFERRED policies that were created with an empty nodemask
294 (local allocation).
295 296MEMORY POLICY REFERENCE COUNTING
297 298To resolve use/free races, struct mempolicy contains an atomic reference
299count field. Internal interfaces, mpol_get()/mpol_put() increment and
300decrement this reference count, respectively. mpol_put() will only free
301the structure back to the mempolicy kmem cache when the reference count
302goes to zero.
303 304When a new memory policy is allocated, its reference count is initialized
305to '1', representing the reference held by the task that is installing the
306new policy. When a pointer to a memory policy structure is stored in another
307structure, another reference is added, as the task's reference will be dropped
308on completion of the policy installation.
309 310During run-time "usage" of the policy, we attempt to minimize atomic operations
311on the reference count, as this can lead to cache lines bouncing between cpus
312and NUMA nodes. "Usage" here means one of the following:
313 3141) querying of the policy, either by the task itself [using the get_mempolicy()
315 API discussed below] or by another task using the /proc/<pid>/numa_maps
316 interface.
317 3182) examination of the policy to determine the policy mode and associated node
319 or node lists, if any, for page allocation. This is considered a "hot
320 path". Note that for MPOL_BIND, the "usage" extends across the entire
321 allocation process, which may sleep during page reclaimation, because the
322 BIND policy nodemask is used, by reference, to filter ineligible nodes.
323 324We can avoid taking an extra reference during the usages listed above as
325follows:
326 3271) we never need to get/free the system default policy as this is never
328 changed nor freed, once the system is up and running.
329 3302) for querying the policy, we do not need to take an extra reference on the
331 target task's task policy nor vma policies because we always acquire the
332 task's mm's mmap_sem for read during the query. The set_mempolicy() and
333 mbind() APIs [see below] always acquire the mmap_sem for write when
334 installing or replacing task or vma policies. Thus, there is no possibility
335 of a task or thread freeing a policy while another task or thread is
336 querying it.
337 3383) Page allocation usage of task or vma policy occurs in the fault path where
339 we hold them mmap_sem for read. Again, because replacing the task or vma
340 policy requires that the mmap_sem be held for write, the policy can't be
341 freed out from under us while we're using it for page allocation.
342 3434) Shared policies require special consideration. One task can replace a
344 shared memory policy while another task, with a distinct mmap_sem, is
345 querying or allocating a page based on the policy. To resolve this
346 potential race, the shared policy infrastructure adds an extra reference
347 to the shared policy during lookup while holding a spin lock on the shared
348 policy management structure. This requires that we drop this extra
349 reference when we're finished "using" the policy. We must drop the
350 extra reference on shared policies in the same query/allocation paths
351 used for non-shared policies. For this reason, shared policies are marked
352 as such, and the extra reference is dropped "conditionally"--i.e., only
353 for shared policies.
354 355 Because of this extra reference counting, and because we must lookup
356 shared policies in a tree structure under spinlock, shared policies are
357 more expensive to use in the page allocation path. This is especially
358 true for shared policies on shared memory regions shared by tasks running
359 on different NUMA nodes. This extra overhead can be avoided by always
360 falling back to task or system default policy for shared memory regions,
361 or by prefaulting the entire shared memory region into memory and locking
362 it down. However, this might not be appropriate for all applications.
363 364MEMORY POLICY APIs
365 366Linux supports 3 system calls for controlling memory policy. These APIS
367always affect only the calling task, the calling task's address space, or
368some shared object mapped into the calling task's address space.
369 370 Note: the headers that define these APIs and the parameter data types
371 for user space applications reside in a package that is not part of
372 the Linux kernel. The kernel system call interfaces, with the 'sys_'
373 prefix, are defined in <linux/syscalls.h>; the mode and flag
374 definitions are defined in <linux/mempolicy.h>.
375 376Set [Task] Memory Policy:
377 378 long set_mempolicy(int mode, const unsigned long *nmask,
379 unsigned long maxnode);
380 381 Set's the calling task's "task/process memory policy" to mode
382 specified by the 'mode' argument and the set of nodes defined
383 by 'nmask'. 'nmask' points to a bit mask of node ids containing
384 at least 'maxnode' ids. Optional mode flags may be passed by
385 combining the 'mode' argument with the flag (for example:
386 MPOL_INTERLEAVE | MPOL_F_STATIC_NODES).
387 388 See the set_mempolicy(2) man page for more details
389 390 391Get [Task] Memory Policy or Related Information
392 393 long get_mempolicy(int *mode,
394 const unsigned long *nmask, unsigned long maxnode,
395 void *addr, int flags);
396 397 Queries the "task/process memory policy" of the calling task, or
398 the policy or location of a specified virtual address, depending
399 on the 'flags' argument.
400 401 See the get_mempolicy(2) man page for more details
402 403 404Install VMA/Shared Policy for a Range of Task's Address Space
405 406 long mbind(void *start, unsigned long len, int mode,
407 const unsigned long *nmask, unsigned long maxnode,
408 unsigned flags);
409 410 mbind() installs the policy specified by (mode, nmask, maxnodes) as
411 a VMA policy for the range of the calling task's address space
412 specified by the 'start' and 'len' arguments. Additional actions
413 may be requested via the 'flags' argument.
414 415 See the mbind(2) man page for more details.
416 417MEMORY POLICY COMMAND LINE INTERFACE
418 419Although not strictly part of the Linux implementation of memory policy,
420a command line tool, numactl(8), exists that allows one to:
421 422+ set the task policy for a specified program via set_mempolicy(2), fork(2) and
423 exec(2)
424 425+ set the shared policy for a shared memory segment via mbind(2)
426 427The numactl(8) tool is packaged with the run-time version of the library
428containing the memory policy system call wrappers. Some distributions
429package the headers and compile-time libraries in a separate development
430package.
431 432 433MEMORY POLICIES AND CPUSETS
434 435Memory policies work within cpusets as described above. For memory policies
436that require a node or set of nodes, the nodes are restricted to the set of
437nodes whose memories are allowed by the cpuset constraints. If the nodemask
438specified for the policy contains nodes that are not allowed by the cpuset and
439MPOL_F_RELATIVE_NODES is not used, the intersection of the set of nodes
440specified for the policy and the set of nodes with memory is used. If the
441result is the empty set, the policy is considered invalid and cannot be
442installed. If MPOL_F_RELATIVE_NODES is used, the policy's nodes are mapped
443onto and folded into the task's set of allowed nodes as previously described.
444 445The interaction of memory policies and cpusets can be problematic when tasks
446in two cpusets share access to a memory region, such as shared memory segments
447created by shmget() of mmap() with the MAP_ANONYMOUS and MAP_SHARED flags, and
448any of the tasks install shared policy on the region, only nodes whose
449memories are allowed in both cpusets may be used in the policies. Obtaining
450this information requires "stepping outside" the memory policy APIs to use the
451cpuset information and requires that one know in what cpusets other task might
452be attaching to the shared region. Furthermore, if the cpusets' allowed
453memory sets are disjoint, "local" allocation is the only valid policy.
454